Dual energy imaging has the potential to add new and powerful diagnostic capabilities to a medical X-ray examination. Unlike an ordinary X-ray image, dual energy image data can be processed to give information about the chemical composition of body tissues. This new information can be used, for example, to enhance the detectability of microcalcifications in mammography or to quantify calcium in chest radiography. Both are important diagnostic indications. In the past, specialized equipment was needed to produce dual energy images. The introduction of computed radiography systems for use in general radiology provides the opportunity addressed by the proposed work. The long-term goal is to develop an energy selective cassette to acquire dual energy data that is compatible with the general purpose computed radiography systems. The cassette will use a unique approach based on active modulation of luminescence gain invented by Aprend Technology. This new approach produces substantially higher quality dual energy data than alternative approaches while maintaining compatibility with general purpose equipment. The Phase I work will show the feasibility of the approach. If Phase I is successful, Phase II will build a prototype and test it clinically.